1. Field of Invention
This invention relates generally to magnetic flowmeters, and more particularly to a flowmeter of this type which includes a ceramic flow tube or spool through which the fluid to be metered is conducted, the ceramic spool being metallized to define the measuring and shield electrodes of the flowmeter.
2. Prior Art
Magnetic flowmeters such as those disclosed in U.S. Pat. Nos. 3,695,104; 3,824,856; 3,783,687 and 3,965,738, are especially adapted to measure the volumetric flow rates of fluids which present difficult handling problems, such as corrosive acids, sewage and slurries. Because the instrument is free of flow obstructions, it does not tend to plug or foul.
In a magnetic flowmeter, an electromagnetic field is generated whose lines of flux are mutually perpendicular to the longitudinal axis of the flow tube or spool through which the fluid to be metered is conducted and to the transverse axis along which the measuring electrodes are located at diametrically-opposed positions with respect to the spool. The operating principles are based on Faraday's law of induction, which states that the voltage induced across any conductor as it moves at right angles through a magnetic field will be proportional to the velocity of that conductor. The metered fluid effectively constitutes a series of fluid conductors moving through the magnetic field; the more rapid the rate of flow, the greater the instantaneous value of the voltage established at the electrodes.
In the Appel U.S. Pat. No. 4,019,366 and in the Schmoock U.S. Pat. No. 4,098,118, instead of small area measuring electrodes in direct contact with the fluid being metered, use is made of electrode assemblies encapsulated in insulation material. Each assembly is formed by a measuring electrode having a large area behind which is a driven shielding electrode of even greater area, the measuring electrodes being isolated from the fluid by a layer of insulation. Each measuring electrode forms one plate of a capacitor whose dielectric is the insulation layer and whose other plate is the fluid, the electrodes acting as a capacitance sensor to detect the voltage induced in the fluid.
A capacitance electrode sensor of this type obviates slurry and galvanic noise problems and is not subject to leakage. Also among the advantages of a capacitance sensor over contact electrodes in a magnetic flowmeter are that the conductivity range of the fluid to be metered may extend down to as low as 0.1 .mu.S/cm or less, and one may use ordinary metals for the electrodes rather than special materials capable of withstanding the adverse effects of corrosive or abrasive fluids in contact with the electrodes.
In the co-pending applications of Appel, et al. Ser. No. 625,205 filed June 27, 1984, Pat. No. 4,539,853, whose entire disclosure is incorporated herein by reference that is disclosed in a magnetic flowmeter which includes a spool formed by a metallizable ceramic through which the fluid to be metered is conducted. The outer surface of the spool is metallized in its central region to define a pair of capacitative metering electrodes at diametrically-opposed positions, an electromagnetic field being established within the spool whereby when the flowing fluid intercepts this field, an emf is induced therein to produce a signal at the metering electrodes which is a function of flow rate. Overlying each metering electrode and projecting from the spool is a ceramic block whose outer surface is metallized to define a shield electrode which covers the metering electrode.
In this Appel et al. arrangement, the ceramic spool is fabricated of aluminum oxide (alumina). This material has a consistent dielectric constant and high resistivity over the broad range of temperatures encountered in flowmeter operations. Alumina also has excellent abrasion and corrosion resistance and thereby can be used with corrosive fluids as well as those which the flow signal must be measured. Each electrode forms one plate of a capacitor whose dielectric is the ceramic on which it is plated and whose other plate is the fluid being metered. Hence the flow signal is measured through a pair of capacitors located in the body of the alumina spool. The smaller the capacitance values of these capacitors, the higher is the resultant electrical impedance.
Since alumina has a dielectric constant of 9, the capacitance of the two electrode capacitors is quite low. The resultant high impedance creates problems in sensing the flow signal induced in the fluid and picked up by the electrodes. As a consequence, it becomes necessary to use relatively complex and expensive pre-amplifier and amplifier stages. And since the amount of noise coming from the pre-amplifiers is a function of their source input impedance, the noise level in this prior arrangement is relatively high and gives rise to difficulties in distinguishing between the flow signal and the noise to provide accurate flow rate readings.